Circuit breakers are widely used in residential and industrial applications for the interruption of electrical current in power lines upon conditions of severe overcurrent caused by short circuits or by ground faults. One of the problems associated with the process of interruption of the current during severe overcurrent conditions is arcing. Arcing occurs between the contacts of circuit breakers used to interrupt the current, which is highly undesirable for several reasons. Arcing causes deterioration of the contacts of the breaker and it causes gas pressure to build up. It also necessitates circuit breakers with larger separation between the contacts in the open position to ensure that the arc does not persist with the contacts in the fully open position.
Prior art devices have used a number of approaches to limit the occurrence of arcing. In heavy duty switchgear, the contacts may be enclosed in a vacuum or in an atmosphere of SF.sub.6. Both of these approaches are expensive. Besides, SF.sub.6 has been identified as a greenhouse gas.
Another approach that has been used to limit the amount of arcing is the use of a resistor connected in parallel with the main contacts of the circuit breaker. Upon opening of the main contacts, current can still flow through the shunt resistor, effectively reducing the amount of arcing in the main contacts. The current flowing through the resistor is less than the short circuit current that would flow through the main contacts in the absence of the resistor, and the opening of a second pair of contacts connected in series with the resistor can be accomplished with less arcing than would occur in the absence of the shunt resistor.
Tanaka et al., (U.S. Pat. No. 5,424,504) teach a circuit breaker in which a resistor-provided UHV breaker has a tank sealing an insulating gas, a main contact and a resistor unit connected in parallel to the main contacts also located in the tank. Mechanisms are provided so that the resistor contact is made before and broken after the main contact is made and broken. The resistor has to be rated to withstand the high currents and temperatures during short circuit conditions.
Khalid U.S. Pat. No. 4,070,641 teaches a current limiting circuit breaker in which the current limiting contacts are in series with the main contacts of a breaker. Opening of the limiting contacts shunts high fault current through the resistor. The resistor is an iron wire resistor with a positive temperature coefficient (PTC) of resistance. The flow of the short circuit current through the resistor heats the resistor, thereby increasing its resistance and limiting the buildup of the short circuit current.
Perkins et al. ("IEEE Transactions on Components, Hybrids, and Manufacturing Technology", v.CHMT-5, A New PTC Resistor For Power Applications, pgs 225-230 (June 1982) describe a PTC resistor that utilizes the metal-insulator solid state transition in (V,Cr).sub.2 O.sub.3. At a transition temperature of 80.degree. C., the resistivity of a ceramic body including (V,Cr).sub.2 O.sub.3 increases to a value 100 times the value at 20.degree. C. They disclose the use of a PTC element for overcurrent protection as a substitute for a bimetallic strip for overcurrent protection. The switch is connected to a PTC element that is shunted by the actuating coil for the switch. During normal operating conditions, the current flows through the PTC resistor. During short circuit conditions, the rapid heating of the PTC resistor leads to an increased resistance and voltage across the PTC resistor, diverting current through the actuating coil which then trips the switch.
Hansson et al. (U.S. Pat. No. 5,382,938) disclose a PTC element that is capable of withstanding short circuit currents without damage, thereby enabling it to be reused. FIG. 1 shows a disclosure by Hansson '938 on the use of a PTC element 22 as an overcurrent protection device for a motor 25. The PTC element is connected in series with a switch 23 and in parallel with an excitation coil 24 that operates the switch 23. An overcurrent in the circuit heats the PTC element 22 and at a certain temperature, its resistance rises sharply. The voltage across the PTC element 22 is then sufficient to cause the excitation coil 24 to trip the switch 23. Hansson et al (WO 91/12643) discloses a more complicated invention for motor and short circuit protection using a PTC element. A switch is connected in series with a tripping circuit consisting of two parallel connected current branches. One of these branches has the excitation coil for the switch while the other branch has two PTC resistors. Overcurrent conditions cause a buildup of voltage across the PTC resistors that then activates the excitation coil for the switch.
Chen (U.S. Pat. No. 5,629,658) discloses a number of devices in which PTC elements are used in conjunction with two or more switches to limit the current under short circuit conditions and thereby reduce the associated arcing.
One problem with PTC resistors is their durability: they have to be designed to withstand the heating that accompanies short circuit currents. This can make their use more expensive unless arrangements are made to limit this heating.
Yet another approach used to reduce arcing in switchgear involves the use of mechanical means to break the arc. Belbel et al. (U.S. Pat. No. 4,562,323) discloses a switch in which an electrically insulating screen is inserted between the contacts during the opening of the contacts. The control of the movement of the screen is obtained by propulsion means separate from those causing the separation of the contacts. Belbel et al. (U.S. Pat. No. 4,677,266) disclose another switch that has an insulating screen that adapts the breaking speed as the current increases. Bratkowski et al. (U.S. Pat. No. 4,801,772) disclose a current limiting circuit interrupted in which an insulating wedge is inserted between the contact arms as they open.
Most of the prior art methods discussed above are addressed towards industrial applications. For residential use, even though the voltage and the loads are smaller, commercially available circuit breakers still have a significant amount of arcing accompanying their operation. The present invention achieves interruption of electrical currents with a reduction in arcing, noise and gas venting. The present invention also reduces the cost and enclosure requirements for residential circuit breakers and increases the switching capacity of a normal relay.
Additionally, the invention improves the operation of circuit breakers when a ground fault is detected without a mechanical linkage between the ground fault circuit and the circuit breaker. For heavy duty and high voltage devices, the invention replaces SF.sub.6 and vacuum switchgear by air circuit breakers while accomplishing many of the objectives listed above.